The present invention generally relates to discrete parts having a useful barrier coating applied to a portion thereof and a method and apparatus for processing parts such as threaded fasteners with such a coating material. More particularly, the invention relates to the deposition of liquid fluorocarbon or hydrocarbon type coating materials in a precise, continuous and high speed manner onto selected surfaces of metal fasteners to form a barrier coating on the fasteners. A particular application of the invention is the application of liquid fluorocarbon coating material to the internal threads of a nut.
In many industries, metal parts are being increasingly exposed to electrodeposition paints, primers and corrosion resistant materials. For example, recent advances in improving the corrosion resistance of automobile bodies have made the use of formulations such as the corrosion resistant coating material sold under the trademark UNIPRIME.RTM., made by PPG Corporation for the treatment of steel structural members, a standard in the industry. Many fastening elements are permanently attached to basic vehicle structural components prior to processing of the components with electrodeposited primers, paints and rust inhibitors. Therefore, any exposed threads of fasteners attached to such vehicle components may become contaminated, making it difficult or impossible to thread such exposed fasteners with a mating fastener for subsequent assembly. The need therefore arose to develop a way of preventing contamination of these exposed fastener threads that would not substantially interfere with the ultimate performance of such fasteners.
The prior art has proposed a variety of coating systems to attempt to solve the problem of resisting corrosion inhibitor build up on the threads of fasteners. Each of these known systems, however, has suffered from some rather substantial drawbacks. Several alternative methods have been proposed for the coating of the threads of internally threaded fasteners including pierce nuts and weld nuts that utilize liquid epoxy paints or other fluorocarbon coating materials that include TEFLON.RTM. and an organic solvent.
In one of the earliest of these known methods, a liquid TEFLON.RTM. coating material containing fluorinated ethylene propylene (FEP) and a solvent was sprayed onto the threads of a nut using a small high pressure nozzle. The fastener was then heated to a temperature of about 450.degree. F for twenty minutes vaporizing the organic solvent and curing the remaining fluorocarbon material. This method had several disadvantages.
First, with the pressurized spraying techniques used by this method, the coating material impacted the sprayed area at relatively high speeds causing bounce back of some of the material and non-uniform coating or coating of undesired surfaces. Second, because the fluid suspension had to be relatively dilute to avoid clogging of the spray nozzle, the coating at times ran off prior to curing. Third, substantial portions of the expensive fluorocarbon were wasted as excess fluid suspension was applied and dripped down or ran off the fastener prior to curing.
Several liquid fluorocarbon coating systems have been devised to address some of these problems, but these solutions have introduced new problems and limitations. U.S. Pat. No. 4,652,468 to Gould et al. discloses a process for high pressure impact coating of threaded openings of fasteners that attempts to avoid the deposition of coating material on any other surfaces of the fastener. This process requires a masking of the surfaces of the nut in order to restrict the coating material from contaminating the outer surfaces of the nut. Additionally, this process required a choked area for drawing any excess coating material from the opening of the nut. The mandrels and seals utilized to mask fastener surfaces other than the threads have a tendency to wear out quickly due to abrasion and solvent attack. Also, the need to index, mask and remove excess material during the coating process of Gould is complicated, expensive and slows processing speeds.
U.S. Pat. No. 4,701,348 to Neville discloses a method of coating the threads of an internally threaded fastener. Neville requires a metering device with a nozzle to be selectively introduced and removed from a succession of internally threaded fasteners. The reciprocating movement of the nozzle necessitates an indexing of the fasteners that stops the flow of fasteners each time coating material is being applied to any single fastener dramatically slowing processing rates. Furthermore, the nozzle has an ultrasonic tip which is vibrated after the metering of a drop of coating material in order to explode the drop and cause a fine mist of the fluid suspension to be sent toward the threads of the nut. Due to the difficulty in metering identically sized drops in succession and exploding them in the exact same manner using an ultrasonic power source, this system often exhibits uneven coating of the fasteners.
Published PCT International Application No. WO8906757 of Prittinen et al. discloses a method and apparatus for coating internally threaded fasteners with materials such as TEFLON.RTM.. This invention provides an indexed flow of fasteners before an application device that introduces a reciprocating rotary probe into each fastener to be coated. The rotary probe has an opening that deposits a layer of coating material on a preselected portion of the threads of each fastener utilizing a combination of pressurized spraying and centrifugal force. The liquid TEFLON.RTM. coating material emitted from this spray probe is difficult to control. This system is incapable of operating at relatively high production rates since it requires fasteners to be indexed and stopped in place during the entire time of application of the coating material.
Other known solutions, such as those taught by U.S. Pat. No. RE33,766 to Duffy et al. have utilized a stream of powdered TEFLON.RTM. material sprayed onto preheated fasteners. Such systems require a great deal of heat to be applied to the fasteners prior to exposing them to a stream of TEFLON.RTM. powder. The heat utilized in raising the temperature of the fasteners to approximately 700.degree. F. or greater can be both expensive to generate and potentially detrimental to the finish or appearance of the subsequently coated fastener. Due to the inherent difficulties of attempting to adhere powdered TEFLON.RTM. or similar material coating materials, this system generally requires all parts to be cleaned, pickled or plated prior to powder application in order to obtain minimal acceptable adhesion. Production rates in such systems are further limited since a reciprocating rotatable nozzle must be introduced and removed into each internally threaded fastener opening and powder pressure and flow through the multiple nozzles of this system is difficult to maintain in a consistent and uniform manner.
Other liquid material delivery systems such as taught in applicant's copending application Ser. No. 08/270,598 filed Jul. 5, 1994, now U.S. Pat. No. 5,672,376 are also known. Such systems feature high speed accurate delivery of liquid materials such as PVC liquids onto a continuously moving succession of preheated parts. Such systems have not contemplated the application of fluorocarbon or TEFLON.RTM. or similiar material barrier coating materials onto the threads of fasteners to prevent electrodeposition of paints or corrosion resistant materials.
Subsequent use of vibratory feed mechanisms to feed fasteners coated with fluorocarbon type material by these prior art systems to assembly machines has sometimes caused loosening of the coating material. Yet a further problem is created by robotic assembly devices that are now frequently being used in many industries. These robotic assembly devices attach fasteners to structural components. There is an increasing desire, however, to utilize fasteners in such devices in the form of a roll of nuts connected by metal filaments, rather than having the nuts individually presented in loose form to the robotic device.
The individual nuts on these rolls often require fluorocarbon barrier coatings on the threaded surfaces thereof. The ability to feed the coated nuts in the form of an interconnected roll can eliminate the aforementioned loosening of the coatings caused by vibratory feed systems. A further drawback of existing prior art devices is that most of the known methods for the application of fluorocarbon type materials cannot accommodate nuts in the form of a roll of nuts connected by metal filaments, other than by removing all of the nuts from the filaments which is prohibited.
It is therefore apparent that there exists an overwhelming need in the art for an improved apparatus and method of coating the threads and/or other portions of a fastener or other discrete object that overcomes the drawbacks of prior liquid and powdered fluorocarbon deposition systems and features increased quality of coating, increased production rates and the ability to alternatively process fasteners presented individually or in the form of a roll of fasteners connected by metal filaments with equal ability.